The monocrystalline silicon that is the starting material for many semiconductor electronic components is commonly prepared by a Czochralski ("CZ") process. In this process, pieces of polycrystalline silicon are placed in a crucible and melted to a liquidous state, thereby creating a melt. A seed crystal having the desired monocrystalline atomic structure is then lowered into contact with the molten silicon. As the seed crystal is slowly extracted from the melt, a monocrystalline ingot is drawn from the melt having the same atomic structure as the seed crystal.
Unfortunately, dislocation defects are generated in the seed crystal due to the thermal shock created as the seed crystal contacts the melt. Unless corrective actions are taken, the dislocation defects can propagate through and multiply in the growing crystal. As known to those skilled in the art, dislocations generally propagate along crystallographic planes. For a silicon seed crystal having a &lt;100&gt; orientation, the dislocations typically propagate along a plane that extends at an angle of 55.degree. from the longitudinal axis of the crystal.
In order to terminate the dislocations prior to propagation through the main body of the crystalline ingot, crystals are typically grown with a neck section extending between the seed crystal and the main body of the crystal. The most common method of eliminating dislocations is known as the Dash method and involves growing a neck having a relatively small diameter and a relatively long length. For example, a neck grown according to the Dash method may have a diameter of between 2 mm and 4 mm and a length between 30 mm and 200 mm. As the neck is grown, the dislocations propagate through the neck toward the interface of the seed crystal and the melt. As a result of the extended length and small diameter of the neck, however, the dislocations terminate at the exterior surface of the neck such that the main body of the crystal is dislocation free ("DF"). The crystal is then expanded in diameter through the shoulder portion to the DF main body. Since there is no easy and reliable method to determine if the dislocations have been terminated, the Dash method generally requires the neck to have a relatively small diameter and an extended length in order to effectively terminate most, if not all, dislocations.
Although the Dash method is widely utilized, the Dash method has several significant disadvantages. For example, the time and expense associated with growing the neck section are non-recoverable since the neck is ultimately discarded as waste. Also, since the entire crystalline ingot is supported during growth by the relatively thin neck section, the maximum mass of a crystalline ingot is limited, typically to approximately 140 kg. Although this weight limit poses productivity and economic problems for crystalline ingots having conventional diameters of 150 mm or 200 mm, even more problems are created by this weight limit as the silicon industry begins to investigate and grow crystalline ingots having diameters of 300 mm or more.
Since it is a generally accepted premise within the industry that dislocations cannot be consistently removed from necks having diameters exceeding 10 mm, recent efforts have focused on other neck growing techniques to overcome the weight and productivity limitations. For example, U.S. Pat. No. 5,628,823 describes a method for growing a single crystalline ingot including a neck having an upper portion, an intermediate portion, and a lower portion. The upper portion has a diameter that tapers from the diameter of the seed crystal to the relatively constant diameter of the intermediate and lower portions. Although the majority of the intermediate and lower portions have diameters greater than 10 mm, dislocation generation is controlled by adjusting the pulling speeds such that even though the upper portion of the neck has dislocations, the lower portion is free of dislocations. Unfortunately, the neck grown according to U.S. Pat. No. 5,628,823 is relatively long, such as 165 mm, 200 mm or greater.
U.S. Pat. No. 5,126,113 describes an apparatus for supporting a single crystalline ingot during growth. Dislocations in the crystalline ingot are eliminated by growing a small diameter neck using the Dash method as described above. A large diameter bulge is then grown between the Dash neck and the conical section of the crystal body. In order to support the weight of the crystalline ingot during growth, the apparatus of U.S. Pat. No. 5,126,113 includes mechanical grips that engage the recessed lower surface of the bulge. Thus, this technique not only requires a Dash neck to be grown, but also requires a large diameter bulge to be grown for gripping by one or more mechanical grips, thereby complicating both the growth process and the pulling apparatus.
Finally, European Patent Application No. EP 0 795 632 discloses a method of manufacturing a seed crystal for use in a CZ crystal growth process in which the seed crystal is modified so as to define, for example, a hollow cylindrical void. The hollow cylindrical void opens through the lower end of the seed crystal that comes in contact with the melt. As described by EP 0 795 632, the cylindrical void assists in reducing the thermal shock by reducing the surface area that comes in contact with the melt. However, the creation of the hollow cylindrical void generally complicates the machining of the seed crystal.
Therefore, notwithstanding prior techniques to grow DF crystalline ingots, a need still exists for an improved technique for growing DF crystalline ingots. In particular, a need exists for improved techniques for growing relatively large and heavy DF crystalline ingots without subjecting the neck of the crystal to excessive stress and without repeatedly adjusting the pulling speed or requiring additional equipment for lifting or otherwise supporting the crystalline ingot during growth.